The need to divide the power of an engine between the tractive effort and power take-off load, such as hydraulics, has been recognized, even where engine speed is maintained independent of ground or travel speed, such as disclosed in U.S. Pat. No. 3,789,943 issued Feb. 5, 1974 to K. W. Kampert and K. E. Houtz. In the patented arrangement, the engine was operated or governed to run at a "constant" speed and ground speed was determined by controlling the slip in a modulated pressure clutch. Various types of pump arrangements, also have been attempted in the prior art. Removal of a portion of the hydraulic load from the engine when the engine is operating at a high speed is not warranted and yet once the engine slows below the speed at which it develops full torque, it is susceptible to being "killed".
It is, therefore, an object of this invention to provide a means for controlling a variable displacement pump driven by an engine which will maintain full pump displacement at engine speeds above peak torque and will maintain minimum pump displacement at engine speeds below peak torque.
It is also an object of this invention to provide such a means which will produce a smooth transition between the aforesaid conditions.
It is a further object of this invention to provide a means for detecting engine speed and controlling the destroking and stroking of a variable displacement pump over a predetermined engine speed range.
It is another object to provide a speed valve which will smoothly and reliably develop and exhaust pressure to and from a servo-cylinder or control ram for a variable displacement pump.